Wireless communications systems are used in a variety of telecommunications systems, television, radio and other media systems, data communication networks, and other systems to convey information between remote points using wireless transmitters and wireless receivers. A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. Transmitters often include signal amplifiers which receive a radio-frequency or other signal, amplify the signal by a predetermined gain, and communicate the amplified signal. On the other hand, a receiver is an electronic device which, also usually with the aid of an antenna, receives and processes a wireless electromagnetic signal. In certain instances, a transmitter and receiver may be combined into a single device called a transceiver.
Transmitters, receivers, and transceivers often include components known as oscillators. An oscillator may serve many functions in a transmitter, receiver, and/or transceiver, including generating a local oscillator signal (usually in a radio-frequency range) for upconverting baseband signals onto a radio-frequency (RF) carrier and performing modulation for transmission of signals, and/or for downconverting RF signals to baseband signals and performing demodulation of received signals.
In many instances, oscillators must be finely tuned to a specific frequency in order for their associated transmitters, receivers, or transceivers to function as desired.
To achieve such fine tuning, transmitters, receivers, or transceivers often employ digitally-controlled oscillators. An example digitally-controlled Pierce oscillator 500, as known in the art, is depicted in FIG. 5. As shown in FIG. 5, oscillator 500 may include a crystal resonator 10 in parallel with an inverter 12. During operation, inverter 12 may be biased in its linear region, thus allowing it to operate as a high gain inverting amplifier. Each terminal of crystal resonator 10 may also be coupled to a plurality of capacitances. For example, capacitors 14a-14e may be coupled to node X of crystal resonator 10 (thus forming one “capacitor bank” of oscillator 500), and capacitors 14f-14j may be coupled to node Y of crystal resonator 10 (thus forming the other “capacitor bank” of oscillator 500). In many instances, the capacitor banks of oscillator 500 may be substantially identical (e.g., capacitors 14a and 14f may be substantially identical, capacitors 14b and 14g may be substantially identical, capacitors 14c and 14h may be substantially identical, capacitors 14d and 14i may be substantially identical, and capacitors 14e and 14j may be substantially identical).
In each capacitor bank of oscillator 500, one or more capacitors 14 (e.g., capacitors 14a and 14f) may be coupled to a ground voltage, while others (e.g., capacitors 14b-14e and 14g-14j) are switched capacitors coupled to a corresponding transistor 16 such that the non-gate terminals of the corresponding transistor 16 are respectively coupled to the corresponding capacitor 14 and a ground voltage. Each such transistor 16 may act a switch, either effectively coupling its corresponding switched capacitor 14 to ground voltage (enabling such capacitor 14), or leaving one terminal of its corresponding switched capacitor 14 floating (disabling such capacitor 14), depending on the voltage applied to the gate terminal of such transistor 16 (e.g., a “high” voltage applied to the gate terminal of transistor 16b will close the switch of transistor 16b providing a path to ground for a terminal of capacitor 14b, while a “low” voltage applied to the gate terminal of transistor 16b will open the switch of transistor 16b leaving a terminal of capacitor 14b floating). Because capacitors 14a and 14f are not switched capacitors 14, capacitors 14a and 14f may be considered as always enabled. Those of skill in the art will appreciate that the frequency of oscillation of oscillator 500 (e.g., the signal waveform characteristic seen at either of nodes X or Y) will be a function of the sum of the capacitances of the capacitors 14 that are provided a path to ground (e.g., those capacitors 14, including capacitors 14a and 14f, that are enabled). Thus, by switching transistors 16b-16e and 16g-16j, the effective capacitances of the capacitor banks may be modified, thus allowing tuning of the frequency of oscillation of oscillator 500.
As depicted in FIG. 5, the gate terminal of each transistor 16 may be coupled to a control module 20. Control module 20 may be configured to, based on a desired frequency of oscillation for oscillator 500, selectively enable one or more of capacitors 14b-14e and 14g-14j in order to achieve an effective capacitance that allows oscillation at the desired frequency. In many instances, capacitors 14 will be enabled such that the effective capacitance of each capacitor bank is approximately equal.
To permit finer granularity in the switched effective capacitance, and thus finer granularity in the oscillation frequency, one or more of the switched capacitors 14 may be regularly enabled and disabled, in a process called dithering. With dithering, certain capacitors 14 (e.g., capacitors 14b and 14g) may be enabled a certain percentage of the time, and disabled otherwise. The effective capacitance of such dithered capacitance 14 is approximately equal to the capacitance of the dithered capacitor times the percentage of time the dithered capacitor 14 is enabled. To provide for periodicity in the enabling and disabling of dithered capacitors 14, the switching transistors 16 (e.g., transistors 16b and 16g) associated with the dithered capacitors 14 may be coupled to a delta-sigma modulator 22 of the control module 20. Based on a desired frequency of oscillation for oscillator 500, delta-sigma modulator 22 may produce a periodic signal with a duty cycle appropriate to enable dithered capacitors 14 for a certain percentage of time, and disabled the dithered capacitors 14 otherwise.
As noted above, oscillator designs often attempt to match the effective capacitance of one capacitor bank to the other capacitor bank. Such matching allows for an oscillation signal on node X and/or node Y to have an approximate 50% duty cycle. It is often desired to also match the effective capacitance when dithering. Accordingly, in traditional approaches, the dithered capacitors 14 in each capacitor bank are enabled and disabled approximately in unison, which is shown in FIG. 5 where the gate terminal of transistor 16b (node A) and gate terminal of transistor 16g (node B) are both coupled to the output of delta-sigma modulator 22. However, such matching of enable signals for switching transistors 16 of dithered capacitors 14 may have undesirable effects. One such undesirable effect is depicted in FIG. 6.
FIG. 6 is a graph showing example signal waveforms for nodes A, B, and X of oscillator 500 depicted in FIG. 5. As shown, signals at nodes A and B may transition approximately in unison. However, the substantially contemporaneous rising and falling edges of such signals may induce glitch 602 into node X, the output of oscillator 500, especially where non-ideal conditions in oscillator 500 cause enable signals on nodes A and B to be slightly out of phase with one another. Because the output of oscillator 500 governs the modulation and demodulation of wireless transmissions in a transmitter, receiver, and/or transceiver, such high-frequency glitch 602 may cause undesired effects.